In an optical head used for performing recording or reproduction on/from an optical disk, focusing adjustment and tracking adjustment are performed by an actuator which drives an objective lens. The objective lens is driven based on the detection of position deviations of a focusing direction and a tracking direction of a light spot irradiated on the optical disk. The detection of the position deviations is carried out by allowing light reflected from the optical disk to fall on a light-receiving element.
When the objective lens shifts in the tracking direction during tracking adjustment, an irradiated position on the light-receiving element, whereon the light is incident, also moves since the light reflected from the optical disk also shifts. The light reflected from the optical disk shifts because the center of the objective lens is displaced away from the center of light emitted by the light source. As a result, even if the tracking position of a light spot is correct, an erroneous tracking error signal is generated.
In order to prevent this from happening, Japanese Laid-Open Publication No. 79943 (1989) (Tokukaisho 64-79943) discloses an optical type position detecting device which is installed in the optical head. In this optical type position detecting device, a second light source (for example) is attached to the actuator and a second light-receiving element is set to a specified position. The second light source is provided to be separate from the light source which is used for recording and reproduction on/from the optical disk, and the second light-receiving element is provided so as to be separate from the light-receiving element which detects the light reflected from the optical disk. A light beam emitted by the second light source forms a light spot on the second light-receiving element and the movement of the actuator is detected by detecting the displacement of the light spot on the second light-receiving element. Using such an optical type position detecting device causes the erroneous tracking error signal which is generated with the displacement of the objective lens along the tracking direction to be cancelled. An accurate tracking error signal is thereby achieved.
As shown in FIG. 14, a knife edge member 5 can, for example, be used in the optical type position detecting device. That is, a light beam 2 having a substantially circular cross section is emitted by a light source 1 (such as a light emitting diode or a laser). The light beam 2 forms a light spot 4 (shown by hatching) on a light-receiving element 3. A portion of the light beam 2 emitted by the light source 1 is interrupted by the knife edge member 5. Here, if for example the light source 1 and the light-receiving element 3 are made immovable and the knife edge member 5 is displaced along an x-axis, the area covered by the light spot 4 changes since the amount of light interrupted by the knife edge member 5 changes as the knife edge member 5 is displaced along the x-axis. Consequently, the change of position of the knife edge member 5 is detected as a change in output of the light-receiving element 3.
A displacement of the light source 1 along the x-axis can likewise be detected as a change in output of the light-receiving element 3 by making the knife edge member 5 and the light-receiving element 3 immovable.
Further, as shown in FIG. 18, a light-receiving element 6 conventionally in use is separated into four light-receiving sections 6a, 6b, 6c and 6d by parting lines 6e and 6f which are respectively parallel to an x-axis and a y-axis. In this case, the light-receiving element 6 is made immovable. As a result, when a light source 1 is moved along the x-axis, outputs of the light-receiving sections 6a and 6d increase and outputs of the light-receiving sections 6b and 6c decrease since a light spot 7 (shown by hatching) formed by a light beam 2 shifts along the x-axis. Thus, a displacement of the light source 1 along the x-axis can be detected by comparing the sum of the outputs of the light-receiving sections 6a and 6d with the sum of the outputs of the light-receiving sections 6b and 6c. Moreover, when the light source 1 shifts along the y-axis, a displacement of the light source 1 along the y-axis can be detected by comparing the sum of the outputs of the light-receiving sections 6a and 6b with the sum of the outputs of the light-receiving sections 6c and 6d. In other words, the light-receiving element 6 which is separated into the four light-receiving sections 6a to 6d can detect two-dimensional displacement.
In the configuration shown in FIG. 14, the light spot 4 becomes semi-circular when a half of the light beam 2 from the light source 1 is interrupted by the knife edge member 5. A region shown by hatching in FIG. 15 indicates a reduction in the area of the light spot 4 when the knife edge member 5, hitherto in the state described above, is displaced by .DELTA.x along the x-axis. .DELTA.x is the displacement of a projection on the light-receiving element 3 of the knife edge member 5.
A region shown by hatching in FIG. 16 indicates a reduction in the area of the light spot 4 when the knife edge member 5 is displaced by .DELTA.x along the x-axis, the knife edge member 5 initially being in a state wherein more than half the light beam 2 is being interrupted by the knife edge member 5.
As is evident from FIGS. 15 and 16, the magnitude of the reduction in the area of the light spot 4 due to a given displacement of the knife edge member 5 varies depending on the position of the knife edge member 5 before the displacement takes place. Accordingly, since the output of the light-receiving element 3 is proportional to the area covered by the light spot 4, the change in the output of the light-receiving element 3, due to a given displacement of the knife edge member 5, differs depending on the initial position of the knife edge member 5. This is due to the fact that the far-field pattern of the light source 1 is circular (or elliptical).
FIG. 17 shows a change in the output of the light-receiving element 3 when the knife edge member 5 is displaced along the x-axis with the light source 1 and light-receiving element 3 fixed so as to be immovable. The vertical axis shows the output of the light-receiving element 3. The horizontal axis shows the position of the knife edge member 5. The position of the knife edge member 5 when the edge of the knife edge member 5 passes through a center of the light beam 2 corresponds to zero on the horizontal axis. As is clear from the diagram, linearity between the output of the light-receiving element 3 and the position of the knife edge member 5 decreases sharply as the knife edge member 5 is displaced away from the center of the light beam 2. In the figure, r is the radius of the light beam 2 on a plane in which the knife edge member 5 lies.
Further, in the configuration shown in FIG. 18, when the light source 1 is displaced along the x-axis with the light-receiving element 6 fixed so as to be immovable, as shown graphically in FIG. 19, linearity is not achieved between the position of the light source 1 along the x-axis and a differential output of the light-receiving element 6 ((the sum of the outputs of the light-receiving sections 6a and 6d)-(the sum of the outputs of the light-receiving sections 6b and 6c)). Moreover, when the light source 1 is displaced along the y-axis with the light-receiving element 6 fixed so as to be immovable, the relationship between the position of the light source 1 along the y-axis and a differential output of the light-receiving element 6 ((the sum of the outputs of the light-receiving sections 6a and 6b)-(the sum of the outputs of the light-receiving sections 6c and 6d)) resembles that shown in FIG. 19, and linearity is similarly not achieved.
As described above, in the conventional optical type position detecting device, the change in the output of the light-receiving element 3 differs even when the magnitude of the displacement of the knife edge member 5 (see FIG. 14) is the same. As a result, accurate detection of the magnitude of displacement becomes difficult. Also, the change in the output of the light-receiving element 6 differs even when the magnitude of the displacement of the light source 1 (see FIG. 18) is the same. In this case too, accurate detection of the magnitude of displacement becomes difficult.
A degree of linearity is achieved between the position of the knife edge member 5 and the output of the light-receiving element 3 when the edge of the knife edge member 5 is in the vicinity of the center of the light beam 2. Furthermore, a degree of linearity is achieved between the position of the light spot 7 and the output of the light-receiving element 6 when the light spot 7 is in the vicinity of the center of the light-receiving element 6. However, in this case, a problem remains in that the detectable range of the magnitude of the displacement of the knife edge member 5 or of the light spot 7 becomes narrow.